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Author: Subject: Fenton type oxidation
Boffis
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[*] posted on 9-1-2021 at 02:18
Fenton type oxidation


Does anyone have any actual experience of the se of Fenton's reagent in oxidation in organic chemistry? I am trying to oxidize xylitol to xylose. I have found an unreferenced claim in a book that states that xylitol is prepared by reducing xylose and that H2O2/Fe2+ can be used to oxidize xylitol back to xylose.

I tried this with 50g of xylitol (033moles), 1g of ferrous sulphate and 380ml of 3% H2O2 (0.33moles) (that's all I have available at the present). The solution turned amber yellow, there was a modest rise in temperature from from 0 to about 8 C. Both solutions had been prechilled to 0C first to keep the temperature low. After stirring for 30 minutes the mixture was evaporated down to 100ml and used directly in the preparation of furfural. The small yield of furfural suggests that oxidation to xylose was incomplete but increasing the ratio of H2O2 to xylitol could lead to over oxidation. an anyone shed any light on the likely optimum yield.

I am working on the assumption that the reaction is;
C5H12O5 + H2O2 -> C5H10O5 + 2H2O
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Sedit
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[*] posted on 9-1-2021 at 22:10


I tried a while back in attempts of closing the ring on Vanillin. Besides there being issues in general with that specific reaction I was attempting what I did learn is that Fentons reagent is a beast and about as easy to control as a Wolverine on Meth. It would sit there, nothing happening, reaction running cold then POOF, you have steam and tar. When I say Poof.... believe me, its poof, gone, vaporized with black residue left behind. Over... and over... and over.

Now, surely if you follow published literature your mileage will vary but paving that road for Fentons reagent on a new substance is a chemist nightmare( or dream if you like seeing things poof away in front of your eyes, whatever gives ya your jollies I guess).

Best advice I can say is start dilute, start cold. Be careful, and stay protected. Remember Elephants toothpaste reaction? I bet Fenton could best that if soap was added. Just to give you an idea how fast this can run away on you.

Good luck. My specific reaction required extreme control, cold dilute conditions... and I still couldn't control it.





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Boffis
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[*] posted on 10-1-2021 at 04:14


Thanks for the information Sedit!!

I have tried Fenton's type oxidations before and had variable and unpredictable results. The ceiling of my lab still bears the scars of my attempt to oxidize benzotriazole to triazole 4,5-dicarboxylic acid with H2O2/HNO3/Fe2+!! Interestingly I eventually managed to tame the reaction enough to get some product but it wasn't the desired dicarboxylic acid; I still haven't identified the product but I now think that it is an N-oxide.

I carried out the oxidation of xylitol with ice cold 3% H2O2. There was no obvious reaction apart from a very slight rise in temperature to about +5C and the mixture turned amber coloured. I have ordered some stronger, 12%, H2O2 and will try again at perhaps 6% and a small excess of H2O2 over stoichiometry.

I also tried Fenton's original experiments with tartaric acid once. Yields of dihydroxytartaric acid ranged from zero to 18%, The desired product is easily isolated because of its extremely insoluble sodium salt. Having got 18% in one experiment I repeated the procedure exactly as before but at twice the scale and then got almost no yield. I can certainly vouch for the "hard to control" label on these procedures.
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stoichiometric_steve
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[*] posted on 10-1-2021 at 04:31


Quote: Originally posted by Sedit  
I tried a while back in attempts of closing the ring on Vanillin


you mean methyl to methylene? how did you envision that to work with two other more readily oxidizable groups?
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Boffis
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[*] posted on 11-1-2021 at 12:20


Tried this again with 25g of xylitol, 220ml of 3% H2O2 that's a higher ratio of H2O2 to xylitol than before; and 2g of ferrous sulphate (e 4x concentration in first attempt. Then distilled as before with salt and HCl but the yield of very poor, about 0.5g furfural. Failure :(
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